What is the difference between null and undefined?
PICTURE THIS: 1, 2, 2, 8
Simple meaning
Undefined means a variable was declared but never given a value, or a missing object property.
WHY — JavaScript instead of guessing?
Why interviewers care about JavaScript:
contrast on JavaScript, not two memorised paragraphs.
the developer, then one case where picking wrong hurts.
Name the idea, why it exists, then one short example.
End with when you use it and one common pitfall.
STEPS — What happens step by step?
Before you speak the answer, walk the interviewer through these steps:
- 1Undefined means a variable
was declared but never given a value, or a missing object property.
- 2Null is an intentional
empty value we assign.
- 3JSON has null but
not undefined, which matters when serializing API payloads.
- 4Give an example
One tiny concrete case you can say aloud.
- 5Common mistake
What juniors usually get wrong.
- 6Close
When you pick this over the alternative.
EXAMPLE — See it in action
Here's a short line you can speak, broken into clear beats:
Note: Adapt this scaffold to your own project — keep it under 60–90 seconds.
Key takeaway
Undefined means a variable was declared but never given a value, or a missing object property. Null is an intentional empty value we assign.